There is atiny black resistor next to the charging port flex cable. Right below the 7 gold dots. Very easy to know off. Underneath the resistor is 6 very small points. If that was to fall off upon removing the cable, it will not read a sim.
also if it is reading sim card, but still "searching" or "no service" (sometimes switches back and forth) the small blue resistor may have been been detached. Or the smaller resistor next to it. Both are located by the signal cable (which is connected to the loud speaker) may need a new motherboard.